a) Field of the Invention
The present invention relates to an automotive headlamp, and more particularly to an automotive headlamp with a reflector which horizontally diverges light rays emitted from the light source, namely, a light-diverging reflector.
b) Description of the Prior Art
In headlamps for motor vehicles, the geometrical shape of the inner reflecting surface of the reflector is determined depending upon the luminous intensity distribution pattern required of the headlamps. The commonest shape is a paraboloidal surface of revolution as shown in FIG. 1. In a headlamp with a reflector of which the inner reflecting surface is part of a paraboloid of revolution, a lamp bulb (not shown) is disposed near the focus F of the reflecting surface 1 shaped as a part of a paraboloid of revolution and the light rays emitted from the light source are reflected at the reflecting surface 1 in directions generally parallel to the optical axis z. The rays are refracted by the multiple light-diverging prisms 2a formed on the inner surface of the outer lens 2 and projected to the right and left from the outer surface of the outer lens at predetermined angle, for example, -.theta.o and .theta.m, formed with respect to the optical axis z. The luminous intensity distribution pattern defined on a test screen by the light rays from the headlamp with the reflecting surface 1 of this type being a part of a paraboloid of revolution is a horizontally long pattern of a relatively uniform brightness extending at angles +.theta.m and -.theta.m to the right and left as shown in FIG. 2. The headlamp of this type is advantageous because its luminous intensity distribution pattern is one required of the automotive headlamps, but it is disadvantageous in that the light loss at the light-diverging prism 2a is large.
FIG. 3 schematically shows the optical system of a headlamp with a so-called light-diverging reflector, already proposed to eliminate the disadvantages of the reflecting surface having the aforementioned form of a paraboloid of revolution. The inner reflecting surface 3 of this reflector reflects the light rays emitted from the light source F and incident upon positions away from the optical axis z in directions away from the optical axis. Namely, at the center of the inner reflecting surface, the rays incident from the light source F are reflected in the direction of the optical axis z while at a position outwardly farther from the optical axis z, the incident rays are reflected in directions of which the angle with respect to the optical axis is larger. The rays incident upon the right and left ends, the incident rays are reflected at angles +.theta.m and -.theta.m respectively, with respect to the optical axis. Also, for effective utilization of the rays emitted directly forward from the light source without being incident upon the reflecting surface 3, a supplemental spherical mirror 4 (indicated with dash line in FIG. 3) may be provided which reflects toward the reflecting surface 3 the rays emitted forward from the light source F, but this arrangement is not advantageous since the directions of the rays reflected near the center of the inner reflecting surface are not sufficiently large with respect to the optical axis and the supplemental spherical mirror 4 will block the light rays. The luminous intensity distribution pattern formed on a test screen by the rays from the headlamp with the reflector having a single reflecting surface of this kind can have a certain angle of vertical divergence at the center of the screen because of the filament size but has extremely small angles of vertical divergence at both the right and left ends, as shown in FIG. 4. So the headlamp of this type cannot illuminate the road surface uniformly.
FIG. 5 schematically illustrates the optical system of a headlamp having a reflector already proposed to eliminate the disadvantages of the aforementioned reflecting surface. The inner reflecting surface of this reflector 5 reflects the rays emitted from the light source F and incident upon positions away from the optical axis z in directions rather parallel to the optical axis. Namely, at the center of the inner reflecting surface, the incident rays from the light source F are reflected at divergent angles m with respect to the direction of the optical axis z. At positions outwardly farther from the optical axis z, the incident rays are reflected in directions of which the angle m with respect to the optical axis is smaller. And at the right and left ends outwardly farthest from the optical axis z, the incident rays are reflected in directions parallel to the optical axis z. The luminous intensity distribution pattern formed on a test screen by the rays from the headlamp having with a reflector having a single reflecting surface of which the reflecting characteristics are as mentioned above can have a certain angle of vertical divergence at both the right and left ends but has an extremely small angle of vertical divregence at the center, as shown in FIG. 6. So, the headlamp of this type cannot illuminate the road surface uniformly.
SUMMARY OF THE INVENTION
The present invention has an object to provide a headlamp having a reflector specially designed to effectively utilize the light rays from the light source for illumination of the road surface.
The present invention has another object to provide an improved headlamp having a light-diverging reflector which can project the rays emitted from the light source uniformly in horizontal directions in a predetermined range of angle
The present invention has still another object to provide an improved headlamp having a light-diverging reflector usable in conjunction with a supplemental reflecting surface to utilize further effectively the rays emitted from the light source.
The above-mentioned objects can be attained by providing a headlamp comprising, according to the present invention, a concave mirror having an inner reflecting surface, a lamp bulb
as light source having the center thereof disposed nearly on the optical axis of the concave mirror and a substantially transparent cover disposed in front of the lamp bulb and covering the front opening of the concave mirror, the inner reflecting surface of the concave mirror being a composite paraboloidal surface of revolution made of parts of multiple different paraboloidal surfaces of revolution taking as common focus a predetermined point on the optical axis and smoothly joined to each other and the lamp bulb being so disposed as to have the center thereof disposed in the vicinity of the common focus. The center axis of each paraboloidal surface of revolution is a straight line passing through the common focus and offset a predetermined angle from the optical axis in a horizontal plane in which the optical axis lies. The inner reflecting surface consists of a first reflecting zone, a second reflecting zone adjoining the first reflecting zone and a third reflecting zone adjoining the second reflecting zone, which are defined depending upon their distances from the common focus. The plural paraboloidal surfaces of revolution belonging to the first reflecting zone reflect or diverge the light rays emitted from the lamp bulb and incident upon positions nearer to the common focus in directions farther from the optical axis. The plural paraboloidal surfaces of revolution included in the second reflecting zone reflect the rays incident from the lamp bulb in directions nearly parallel to the optical axis. And the plural paraboloidal surfaces of revolution belonging to the third reflecting zone reflect or converge the rays emitted from the lamp bulb and incident upon positions farther from the common focus in directions nearer to the optical axis. All the rays reflected at the first, second and third reflecting zones, respectively, are directed parallelly to a horizontal plane in which the optical axis lies, but since the angles of these directions with respect to the optical axis are different depending upon their distances from the common focus, the luminous intensity distribution pattern can have an ample amount of light and the pattern can be extended nearly uniformly from its center horizontally to the right and left, and also the light amount can be adjusted by changing the geometrical shape of each paraboloidal surface of revolution and the areas of the reflecting zones. Therefore, the rays emitted from the lamp bulb can be utilized most effectively for illumination of the road surface.